An investigation is underway into threatening letters sent to parents of children at Christchurch's Mt Pleasant School, that demand money and threaten to harm the children.

Two families have received the letters in the last week.

Detective Inspector Greg Murton said while there's no proof the threats are genuine, police are not taking any chances.

"We're treating this matter very seriously, and we have a team of detectives working to identify the author of these letters. We have increased police patrols in the area and also put additional security in place for the families involved."

Greg Murton won't speculate on how many families might have received letters.

"While there is no evidence of a genuine threat towards any children at Mt Pleasant School, parents have been advised of the existence of the letters. We are confident that the steps taken will ensure the safety of the children involved."

Principal Chris Nord said the school's communicated with parents to ensure they're aware of the situation and will keep them updated.

He's urged anyone in the community with information or who has received a letter to to come forward.